This invention relates to call restriction arrangements for telephone communication systems and, in particular, to an improved common control circuit arrangement for screening both rotary and multifrequency calling digits.
Some call screening arrangements utilize a combined register-check circuit associated with an outgoing line on every call. The circuit continuously monitors the line, records each dialed digit, detects invalid dial codes, and effects the release of the calling connection if the dialed digits represent a restricted code. Inasmuch as these circuits remain connected to the line during dialing, long holding times are experienced, particularly on the subscriber-originated calls from a private branch exchange (PBX). In order to insure that sufficient numbers of these circuits are available to screen every call from a busy PBX, it is oftentimes necessary to provide almost one such circuit for every trunk.
As a solution to the above problem, a system has been developed in which each telephone line is equipped with a dial pulse register for storing the first three dialed digits. A common check circuit is then used to scan the digits registered with each line and when an invalid code is recorded in any register the common check circuit operates to restrict the call associated with that line. Such an arrangement is described in U.S. Pat. No. 3,821,485 issued to R. J. Harrington, A. E. Low and M. J. Sturtevant on June 28, 1974.
Although the above-mentioned Harrington et al patent is an advance in the art, one problem remains, namely, that the system operates only on rotary dialed digits and is not suited for use with multifrequency-dialed calls. This result follows from the fact that although it is a relatively easy operation to decode sequential rotary-dialed digits which are basically a stream of electrical pulses, it is a much more complicated matter to decode and translate electrical signal frequencies into dialed digit equivalents. Thus, the decoding and translating equipment which would be necessary on a per line basis if the Harrington et al. teaching were to be used with multifrequency dialing becomes prohibitively expensive.
Compounding the problem further is the fact that any line can be used for both rotary-dialed calls and for multifrequency-dialed calls, thereby requiring both a dial pulse register and a multifrequency pulse receiver to be associated with each line. Further compounding the problem is the fact that central office common control equipment can be arranged to process a call which is partially rotary dialed and partially multifrequency dialed.
Accordingly, a need exists in the art for a toll restrictor circuit capable of handling both rotary-dialed and multifrequency-dialed calls as well as being able to respond to calls partially dialed with either dialing scheme.